Lithium ion secondary battery is an energy storage device with a high energy density, which can be charged and discharged by reversible conversion between chemical energy and electric energy, and is basically comprised of an anode, a cathode, a separator, and an electrolyte. It has been widely applied to small-sized electronic devices, for example, mobile phones, notebook computers, and the like.
In recent years, in order to deal with environmental issues, high oil prices, and energy efficiency and storage, application of the lithium ion batteries has been rapidly expanded to hybrid electric vehicles (HEV), plug-in EV, e-bikes, and energy storage systems (ESS).
Although the lithium ion battery is a stable electrochemical device insulated by a separator, a short circuit between an anode and a cathode may be caused by internal or external abnormalities of the battery or by shocks and there is a possibility of heating and explosion. Therefore, ensuring thermal/chemical stability of the separator as insulator is the most important consideration.
Further, in recent years, along with an increasing demand for high capacity and high power output of a lithium secondary battery, stability of a battery, that is, stability in generation of explosion and fire, becomes more important. In order to ensure stability of a battery, factors such as voltage, current, impedance, temperature, and the like of a battery cell are generally controlled by an electronic protection circuit or a battery management system (BMS), thereby responding to abnormalities, such as over charge and over current, of the battery.
A polyolefin-based separator commercially used in lithium secondary batteries is a porous film that prevents a short circuit between an anode and a cathode and provides pores serving as a passage of lithium ions. Polyolefin-based separators manufactured by a wet method and a dry method have been widely used commercially.
The wet method is a method including melt-mixing inorganic particles or oil components with polyolefin, extruding the resultant using an extruder to prepare a sheet, forming a thin film by simultaneous or successive biaxial stretching using a roller or a tenter, and extracting the inorganic particles or oil components using a solvent to produce a porous film. This method is commercially used to form a film mainly using polyethylene (PE) among polyolefins. On the other hand, the dry method is a method for forming a porous film by melt-extruding a resin and then stretching the resultant with a roller or a tenter without using an organic solvent, and generally uses polypropylene as the resin and may use polyethylene as necessary.
Herein, in a process for preparing a porous separator, except a non-woven separator, a polyolefin porous separator in which a porous substrate is generally prepared by a film stretching process, cannot avoid a change in volume such as contraction or fusion when a temperature of a battery is increased to 100° C. or more due to an internal or external stimulus, and, thus, an electrical short circuit between an anode and a cathode may cause explosion. Further, if the separator is broken due to dendrite growth within the battery, an internal short circuit may induce explosion of the battery.
There is disclosed a coated separator in which in order to suppress thermal contraction caused by high temperature and instability of the battery caused by dendrite, inorganic particles together with a binder are coated on one or both surfaces of the substrate of the porous separator, and, thus, the inorganic particles suppresses a contraction rate of the substrate and the separator is more stable due to the inorganic coating layer.
In this case, if an organic/inorganic coating layer applied to the porous substrate is not uniformly coated on the porous substrate, when a secondary battery is assembled or within the assembled battery, a part of the inorganic coating layer can be easily separated due to coating defects on the surface. Such separation can decrease stability of the battery. Therefore, a coating system for more uniform organic/inorganic coating is needed to form a uniform inorganic coating layer and ensure an excellent battery property.
As a conventionally well-known technology for a separator with an organic/inorganic coating, a process for preparing a coated porous separator by coating organic/inorganic slurry (PVDF-CTFE/BaTiO3 or PVDF-CTFE/Al2O3) with an organic solvent is described in Korean Patent No. 0775310. This process is identical with a conventional electrode solution molding process using a great amount of an N-methylpyrrolidone (NMP) solvent or acetone as a dispersion medium.
Typically, an organic solvent dissolves a binder (PVDF-CTFE) to provide an excellent adhesive property between inorganic particles in powder form when it evaporates. Slurry prepared from a solution of binder in an organic solvent provides interconnectivity between a porous substrate and an organic/inorganic coating layer, and between inorganic particles within the inorganic coating layer. The components connected as such can endure contraction of a porous separator caused by heating and an external physical event without losing the interconnectivity when a battery is assembled and operated.
However, a coating method based on a binder composition soluble in an organic solvent has some problems. Firstly, a binder soluble in an organic solvent is formed into gel as the organic solvent volatilizes during a drying process, resulting in generation of solvent-impermeable spaces and non-uniformity of an organic/inorganic coating layer, and, thus a battery property may deteriorate. In order to overcome this problem, the binder needs to undergo a secondary drying process in a vacuum at a glass transition temperature Tg or higher. If a residual solvent is present in a product due to insufficient drying, a part of the binder is dissolved and gel may be formed. Thus, if a surface of the coating layer becomes sticky, dust from the outside or unnecessary particles may adhere thereto and a defect rate of products may be increased due to adhesion between coating layers or with a substrate when a product is wrapped. Secondly, if a concentration of the binder in the slurry is high, a viscosity of the slurry is highly increased, which makes it difficult to prepare an organic/inorganic complex layer of a thin film. Further, if a concentration of the binder in the slurry is increased, since a boiling point of the slurry is high, a drying process requires a high temperature. Thirdly, if a low viscosity of the slurry is maintained, an adhesive strength between inorganic particles and a porous substrate or between inorganic particles is decreased, and, thus the inorganic particles are easily separated. Fourthly, in a process based on an organic solvent, a dry zone of a drying line is elongated due to a critical explosion limit in drying, and, thus, it is difficult to improve a processing speed. Fifthly, since an organic solvent has volatility, from the moment the slurry is exposed to the external environment, the organic solvent continuously volatilizes. Therefore, a concentration and a rheological property of the slurry are changed due to evaporation of the solvent during the slurry is prepared, transferred, and coated, which may affect a coating quality of a final product. Sixthly, when a coated separator is prepared, risk factors for safety, healthy, and environment are inherent. Since an organic solvent has toxicity, inflammability, and volatility due to its characteristics, in order to lower the risks due to the organic solvent and reduce environmental pollution, the organic solvent needs to be specially prepared and managed. Therefore a preparation method of the organic/inorganic coating separator using a binder soluble in an organic solvent has limitations in view of characteristics of a battery and characteristics of a preparing process.
Meanwhile, Japanese Patent Laid-open Publication No. 2004-227972 describes a method for preparing a coating separator including a water-soluble polymer and fine particles. According to this method, slurry in which alumina particles having average size of 13 nm are dispersed in carboxymethyl cellulose (CMC) aqueous solution, is casted on a polyethylene porous film to form an organic/inorganic complex layer, thereby preparing a coated separator.
However, considering that a case where a separator is coated using only carboxyl methyl cellulose without using alumina particles and a case where a separator is coated using alumina particles are similar in dimensional stability, an effect of controlling a thermal dimensional stability by alumina particles in a separator is not confirmed.
Further, as described above, if small alumina particles of 0.1 μm or less are used, particle dispersion stability in the slurry is decreased, which may induce instability of coating. Fine particles of smaller size than pores in a porous separator infiltrate into the pores and block the pores. Thus, after coating, permeability of the separator is sharply decreased.
As a method for improving an adhesive strength between a porous substrate and a coating layer, Korean Patent Laid-open Publication No. 2012-0052100 describes a technology for preparing a separator having two coating layers, including: forming an organic/inorganic complex layer by casting a slurry in which styrene-butadiene rubber (SBR) and carboxyl methyl cellulose (CMC) are dissolved in acetone as an organic solvent, onto a polyethylene porous film; and carrying out an electric radiation of a polymer solution thereon. If an organic/inorganic complex layer is formed by this method, the above-described problems of the coating method using the organic solvent also occur. When a coating separator is formed in three layers by radiation onto an inorganic coating layer in order to solve the problem caused by separation of inorganic substances due to a low adhesive strength with a substrate, since a film is formed by the radiation, it is difficult to overcome the problem of a thickness control of a coating layer, and, thus this technology does not satisfy a recent trend in which a coating separator is demanded to be thinner. Further, due to low uniformity in pore, a current cannot be uniformly distributed but concentrated on a single point when being applied to a battery, and, thus, partial heating, degradation, and explosion may occur. Therefore, it does not provide a fundamental technical suggestion of an organic/inorganic coating separator.
In order to solve a problem that coating materials are easily separated due to a low adhesive strength between a porous substrate and a coating layer when a porous separator having an organic/inorganic coating is prepared by the above-described prior methods, Korean Patent No. 1125013 describes a method for preparing a cross-linked ceramic-coated separator using a water-soluble ionic polymer. This method also uses an ionic polymer which can be dissolved in water, but the ionic polymer is not dispersed in water but completely dissolved in water, and, thus, it cannot avoid confinement of the solvent. Since the amount of dimethylacetamide used as an organic solvent is 15 times more than that of water, it does not provide a fundamental suggestion of a coating method using water. In order to induce chemical crosslinking after coating for the purpose of improving an adhesive strength with a substrate, a crosslinking agent and an initiator need to be added together with the organic solvent during a preparation process of slurry, and during a drying process, a heat or UV treatment for 20 hours or more is essentially required. However, if a crosslinking agent and an initiator are added to a slurry solution, before the slurry is applied to the porous substrate, the slurry is partially cross-linked by itself by heat and energy externally applied while a coating solution is stored and transferred, resulting in solidification of the slurry. Thus, uniformity of the coated separator is finally decreased. Further, since a heat treatment and a UV treatment requiring a long time are needed even during a drying process, productivity may be very limited, and a porous substrate of a thin film may be damaged due to high temperature/high energy during the drying process, which may cause deterioration in properties and air permeability of a separator.